Such grips are conventionally known, for example, from Japanese Utility Model Publication No. 7-48463 and the like. As shown in FIGS. 37 and 38, the grip is of a structure in which a flexible printed wiring heater circuit 190 (which will be referred to as an FPC heater hereinafter) is integrally embedded, in a mode in which it has been wound around a semi-cylindrical inner piece 33' made of a synthetic resin, in a cylindrical grip body 35' made of a rubber and capable of being fitted into a pipe-shaped steering handlebar 26. A feeder cord (not shown) is soldered to a lead portion 190a of the FPC heater 190 led out of the grip body 35'.
In producing the grip body 35', the following steps are sequentially conducted as shown in FIG. 39: a step of assembling the inner piece 33' having the FPC heater 190 wound therearound to a core 191; a step of winding a sheet 192 of a raw rubber around the FPC heater 190; a step of bringing dies 193 and 194 in a state in which the resulting sheet has been accommodated therein, into close contact with each other to clamp them; a step of pouring a molten rubber into the dies 193 and 194 at a point when the temperature has reached a level at which the raw rubber sheet 192 within the dies 193 and 194 is molten, thereby forming the grip body 35'; and a step of removing the formed grip body 35' out of the dies 193 and 194 at a point when the rubber within the dies 193 and 194 has been vulcanized, and then cooling the removed grip body 35' by leaving it to stand for a predetermined time.
In the above heater-installed grip, however, the inner piece 33' made of the synthetic resin is formed at a circumferentially constant and relatively large wall thickness. For this reason, the flexibility of the grip body 35' is small, and in press-fitting the steering handlebar 26 into the grip body 35', the grip body 35' cannot follow the movement of the steering handlebar 26 which is press-fitted and hence, it is difficult to press-fit the steering handlebar 26 into the grip body 35'. If the steering handlebar 26 is forcibly press-fitted into the grip body 35', there is a possibility that the inner piece 33' may be cracked, resulting in a very poor assemblability of the grip body 35' to the steering handlebar 26.
Therefore, if the inner piece 33' is formed at a relatively small wall thickness, the flexibility of the grip body 35' is increased, whereby the assemblability of the grip body 35' to the steering handlebar 26 is enhanced and the possibility of cracking of the inner piece 33' is eliminated. However, during clamping of the dies in forming the grip body 35', the inner piece 33' may be cracked in some cases, because it cannot withstand the clamping.
The conventional inner piece 33' suffers from the following problem: Since the conventional inner piece 33' is semi-cylindrical, the rubber layer is cooled and shrunk in the course of cooling of the grip body 35' after being removed from the dies 193 and 194, especially, a rubber layer region a opposed to the inner piece 33' is shrunk, and the circumferentially opposite ends b, b of the inner piece 33' are deformed in a direction to approach each other by such shrinking force and as a result, the grip body 36' is warped.
An electric power for heating a heater in such a heater-installed grip by a heater is supplied from a battery mounted on a vehicle. The capacitor of the battery is limited and hence, it is desirable that the grip is effectively warmed by an electric power as small as possible. In the conventional heater-installed grip, however, the FPC heater is disposed in a uniformly dispersed manner in the grip body and is of a construction to uniformly warm the entire grip. Therefore, to warm the grip up to a comfort temperature, there is a possibility that the amount of power consumed may be increased to produce the running-out of the battery. If the amount of power consumed is suppressed, the rising of the temperature of the grip is insufficient.
The conventional heater control device is of an arrangement in which as shown in FIG. 40, heaters 190.sub.L and 190.sub.R mounted respectively in the left and right handlebar grips of the steering handlebar in the motorcycle are connected in series; a switch 195 is mounted between one of the heaters 190.sub.L and the battery 90, and a variable resistor 196 is connected in series to the other heater 190.sub.R, so that the electric current flowing through the heaters 190.sub.L and 190.sub.R is regulated and the power consumed in the heaters 190.sub.L and 190.sub.R is controlled by varying the resistance value of the variable resistor 196, thereby maintaining the handlebar grips at a suitable temperature.
However, there is a problem that the power consumed in the variable resistor 196 is relatively large, and the battery 90 cannot be utilized efficiently. Especially, in a case of the grip heater mounted in the handlebar in the motorcycle or the like, the power consumed in the heaters is large, because it is in direct contact with the open air, and the power consumed in the variable resistor is increased because of a large range of variation in temperature of the open air. This makes it difficult to utilize the battery efficiently.
The warming of the grip by the heaters installed in the grip for the vehicle is attended on the traveling of the vehicle. To have a preference of the traveling of the vehicle, it is required that the supplying of the power from the battery to the heaters should be stopped when the power source voltage of the battery has been dropped down to a value equal to or lower than a voltage required at a minimum for driving a starter motor. Even in the above conventional heater-installed grip, the control of the supplying of the power to the heater is carried out in the above manner. However, if the timing to stop the supplying of the power to the heaters is established so that the supplying of the power is stopped at a voltage threshold value larger than required to reliably ensure a minimum power source voltage required in the battery, a state in which the grip can be warmed by the heaters is extremely limited. If the voltage threshold value is too small, the supplying of the power to the heaters is conducted even if the voltage becomes equal to or smaller than the minimum power source voltage required in the battery due to an error of detection of the voltage or a drop of the voltage from the battery to a detected portion, whereby it may be difficult to start the vehicle. Therefore, it is desirable that the voltage threshold value for determining the timing for stopping the supplying of the power to the heaters is set at a necessary minimum value in consideration of the voltage detection error and the voltage drop.
If the supplying of the power to the heaters is stopped, the power source voltage of the battery is slightly risen. If the supplying of the power to the heaters is controlled in an ON/OFF manner at a constant voltage threshold value, turning-ON/OFF of the supplying of the power to the heater is frequently repeated to produce a control hunching. Therefore, the grip cannot be stably warmed, and a good warm-feeling is not obtained.
In mounting of the grip heater unit comprised of a heater installed in a grip of a steering handlebar and a control device for controlling the heater to a vehicle such as a motorcycle, if the grip heater unit is assembled exclusively for the vehicle as manufactured, then a plus-side connecting wire is connected directly to a plus side of the battery, and minus-side connecting wire is connected directly to a minus side of the battery. In this case, there is particularly not a problem arisen. However, if the grip heater unit is applied to a commercially available vehicle, or the application thereof to a large number of types of vehicles is considered, it is desirable that the connection of the plus-side connecting wire and the minus-side connecting wire extended from the grip heater unit to the plus side and the minus side of the battery can be easily performed for any type of the vehicles. Moreover, the heater is installed in the grip of the turnable steering handlebar, and it is necessary to avoid the arising of problems in the reliability and the durability of the connection structure as a result of twisting of the connecting wires due to the connection of the plus-side and minus-side connecting wires.